Physiological regulation and disorders of phosphate metabolism--pivotal role of fibroblast growth factor 23

FGF23-related hypophosphatemic rickets preceding the onset of systemic lupus erythematosus: A juvenile case

Key Clinical Message

This case report describes the clinical course of a juvenile female with FGF23-related hypophosphatemic rickets preceding the onset of SLE. Our study demonstrates the possibility of hypophosphatemic rickets as an early symptom of SLE.

Abstract

Fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets is observed in both genetic and acquired disorders. Various reports describe FGF23-related hypophosphatemia with systemic lupus erythematosus (SLE), although FGF23-related hypophosphatemia preceding the onset of SLE has never been described. Here, we report the case of a 9-year-old female with FGF23-related hypophosphatemic rickets preceding the onset of SLE. The patient presented to us with arthralgia in the lower extremities and abnormality of gait lasting for 8 months. She was diagnosed with FGF23 hypophosphatemic rickets due to the presence of hypophosphatemic rickets symptoms and high serum levels of FGF23. Additional examination excluded hereditary diseases and tumor-induced osteomalacia. Three months after diagnosis of FGF23-related hypophosphatemic rickets, she developed nephritis and was diagnosed with SLE. She was treated with prednisolone, hemodialysis, and disease-modifying drugs, as well as oral sodium phosphate to improve hypophosphatemia. Serum anti-double-stranded DNA antibody (dsDNAab) and plasma tumor necrosis factor-α (TNF-α) were elevated at FGF23-related hypophosphatemic rickets diagnosis. During the clinical course, serum FGF23 correlated with dsDNAab and TNF-α serum levels, which are involved in SLE disease activity. In this case, FGF23-related hypophosphatemic rickets without hereditary diseases or tumor-induced osteomalacia occurred before the appearance of juvenile SLE symptoms, and serum FGF23 represented disease activity in SLE.

Sex-dependent differences in the genomic profile of lingual sensory neurons in naïve and tongue-tumor bearing mice

Mechanisms of sex-dependent orofacial pain are widely understudied. A significant gap in knowledge exists about comprehensive regulation of tissue-specific trigeminal sensory neurons in diseased state of both sexes. Using RNA sequencing of FACS sorted retro-labeled sensory neurons innervating tongue tissue, we determined changes in transcriptomic profiles in males and female mice under naïve as well as tongue-tumor bearing conditions Our data revealed the following interesting findings: (1) FACS sorting obtained higher number of neurons from female trigeminal ganglia (TG) compared to males; (2) Naïve female neurons innervating the tongue expressed immune cell markers such as Csf1R, C1qa and others, that weren't expressed in males. This was validated by Immunohistochemistry. (3) Accordingly, immune cell markers such as Csf1 exclusively sensitized TRPV1 responses in female TG neurons. (4) Male neurons were more tightly regulated than female neurons upon tumor growth and very few differentially expressed genes (DEGs) overlapped between the sexes, (5) Male DEGs contained higher number of transcription factors whereas female DEGs contained higher number of enzymes, cytokines and chemokines. Collectively, this is the first study to characterize the effect of sex as well as of tongue-tumor on global gene expression, pathways and molecular function of tongue-innervating sensory neurons.

Sex-dependent Differences in the Genomic Profile of Lingual Sensory Neurons in Naïve and Tongue-Tumor Bearing Mice

Mechanisms of sex-dependent orofacial pain are widely understudied. A significant gap in knowledge exists about comprehensive regulation of tissue-specific trigeminal sensory neurons in diseased state of both sexes. Using RNA sequencing of FACS sorted retro-labeled sensory neurons innervating tongue tissue, we determined changes in transcriptomic profiles in males and female mice under naïve as well as tongue-tumor bearing conditions Our data revealed the following interesting findings: 1) Tongue tissue of female mice was innervated with higher number of trigeminal neurons compared to males; 2) Naïve female neurons innervating the tongue exclusively expressed immune cell markers such as Csf1R, C1qa and others, that weren't expressed in males. This was validated by Immunohistochemistry. 4) Accordingly, immune cell markers such as Csf1 exclusively sensitized TRPV1 responses in female TG neurons. 3) Male neurons were more tightly regulated than female neurons upon tumor growth and very few differentially expressed genes (DEGs) overlapped between the sexes, 5) Male DEGs contained higher number of transcription factors whereas female DEGs contained higher number of enzymes, cytokines and chemokines. Collectively, this is the first study to characterize the effect of sex as well as of tongue-tumor on global gene expression, pathways and molecular function of tongue-innervating sensory neurons.

FGF23 functions and disease

The main function of fibroblast growth factor 23 (FGF23) is the regulation of phosphate metabolism through its action on the sodium-dependent phosphate cotransporters in the proximal renal tubules. Additionally, FGF23 interacts with vitamin D and parathyroid hormone in a complex metabolic pathway whose detailed mechanisms are still not clear in human physiology and disease. More recently, research has also focused on the understanding of mechanisms of FGF23 action on organs and system other than the kidneys and bone, as well as on its interaction with other metabolic pathways. Collectively, the new evidence are successfully used for the clinical evaluation and management of FGF23-related disorders, for which new therapies with many potential applications are now available.

Population Pharmacokinetics and Pharmacodynamics of Burosumab in Adult and Pediatric Patients With X-linked Hypophosphatemia

Burosumab is a fully human monoclonal antibody against fibroblast growth factor 23, which has been approved to treat X‐linked hypophosphatemia (XLH) in adult and pediatric patients. The present work describes the pharmacokinetics (PK) of burosumab and the pharmacokinetic‐pharmacodynamic (PK‐PD) relationship between burosumab and serum phosphorus in adult and pediatric patients with XLH. A total of 2844 measurable serum concentrations of burosumab and 6047 measurable serum concentrations of phosphorus in 277 subjects from 9 clinical studies were included in the population PK and PK‐PD modeling. The serum concentration of burosumab following a subcutaneous administration was well described by a population PK model comprising a first‐order absorption, 1‐compartmental distribution, and a linear elimination. The relationship between serum burosumab and serum phosphorus was adequately described by a sigmoid maximal efficacy model. Body weight was the only covariate associated with PK and PK‐PD parameters. No other intrinsic factors affected PK or PK‐PD relationship in adult and pediatric patients with XLH. Further simulations helped to guide the dosing regimen of burosumab in adult and pediatric patients with XLH including age groups with no clinical data.

Validation of the Radiographic Global Impression of Change (RGI-C) score to assess healing of rickets in pediatric X-linked hypophosphatemia (XLH)

BACKGROUND

Rickets is a primary manifestation of pediatric X-linked hypophosphatemia (XLH) - a rare progressive hereditary phosphate-wasting disease. Severity is quantified from radiographs using the Rickets Severity Scale (RSS). The Radiographic Global Impression of Change (RGI-C) is a complementary assessment in which a change score is assigned based on differences in the appearance of rickets on pairs of radiographs compared side by side.

OBJECTIVE

The current study evaluated the reliability, validity, and sensitivity to change of the RGI-C specifically in pediatric XLH.

METHODS

The reliability, validity, and sensitivity to change of the RGI-C were evaluated using data from two studies in pediatric XLH (113 children aged 1-12 years) in which burosumab treatment significantly improved rickets severity. Intra-rater and inter-rater reliability were assessed by three pediatric radiologists.

RESULTS

Intra-rater reliability for RGI-C global score was >90% for agreement within 1 point, with weighted kappa values >0.5, indicating moderate to almost perfect agreement. Inter-rater reliability was also >90% (0.47-0.52 for all reader pairs; moderate agreement). The RGI-C global score showed significant relationships with changes from baseline to week 64 in serum phosphorus (r = -0.397), alkaline phosphatase (-0.611), total RSS (-0.672), standing height (0.268), and patient-reported global functioning (0.306) and comfort/pain functioning (0.409). Based on standardized response means, RGI-C global scores were sensitive to change in RSS, differentiating between those considered improved and greatly improved. Results for validity and sensitivity to change were similar for the RGI-C wrist, knee, and standing long leg scores.

CONCLUSION

The RGI-C is a reliable, valid, and sensitive measure in pediatric XLH, and complementary to the RSS.

FGF23: A Review of Its Role in Mineral Metabolism and Renal and Cardiovascular Disease

FGF23 is a hormone secreted mainly by osteocytes and osteoblasts in bone. Its pivotal role concerns the maintenance of mineral ion homeostasis. It has been confirmed that phosphate and vitamin D metabolisms are related to the effect of FGF23 and its excess or deficiency leads to various hereditary diseases. Multiple studies have shown that FGF23 level increases in the very early stages of chronic kidney disease (CKD), and its concentration may also be highly associated with cardiac complications. The present review is limited to some of the most important aspects of calcium and phosphate metabolism. It discusses the role of FGF23, which is considered an early and sensitive marker for CKD-related bone disease but also as a novel and potent cardiovascular risk factor. Furthermore, this review gives particular attention to the reliability of FGF23 measurement and various confounding factors that may impact on the clinical utility of FGF23. Finally, this review elaborates on the clinical usefulness of FGF23 and evaluates whether FGF23 may be considered a therapeutic target.

FGF23, Frailty, and Falls in SPRINT

BACKGROUND/OBJECTIVES

Chronic kidney disease (CKD) is associated with frailty. Fibroblast growth factor 23 (FGF23) is elevated in CKD and associated with frailty among non-CKD older adults and individuals with human immunodeficiency virus. Whether FGF23 is associated with frailty and falls in CKD is unknown.

DESIGN

Cross-sectional and longitudinal observational study.

SETTING

Systolic Blood Pressure Intervention Trial (SPRINT), a randomized trial evaluating standard (systolic blood pressure [SBP] <140 mm Hg) versus intensive (SBP <120 mm Hg) blood pressure lowering on cardiovascular and cognitive outcomes among older adults without diabetes mellitus.

PARTICIPANTS

A total of 2,376 participants with CKD (estimated glomerular filtration rate [eGFR] <60 mL/min/1.73 m² ).

MEASUREMENTS

The exposure variable was intact FGF23. We used multinomial logistic regression to determine the cross-sectional association of intact FGF23 with frailty and Cox proportional hazards analysis to determine the longitudinal association with incident falls. Models were adjusted for demographics, comorbidities, randomization group, antihypertensives, eGFR, mineral metabolism markers, and frailty.

RESULTS

After adjustment, the odds ratio for prevalent frailty versus non-frailty per twofold higher FGF23 was 1.34 (95% confidence interval [CI] = 1.01-1.77). FGF23 levels in the highest quartile versus the lowest quartile demonstrated more than a twofold increased fall risk (hazard ratio [HR] = 2.32; 95% CI = 1.26-4.26), and the HR per twofold higher FGF23 was 1.99 (95% CI = 1.48-2.68).

CONCLUSION

Among SPRINT participants with CKD, FGF23 was associated with prevalent frailty and falls.

A Land of Controversy: Fibroblast Growth Factor-23 and Uremic Cardiac Hypertrophy

Cardiac hypertrophy is a common feature in patients with CKD. Recent studies revealed that two phosphate regulators, fibroblast growth factor-23 and α-Klotho, are highly involved in the pathophysiologic process of CKD-induced cardiac hypertrophy. With decreasing renal function, elevated fibroblast growth factor-23 and decreased α-Klotho may contribute to cardiac hypertrophy by targeting the heart directly or by inducing systemic changes, such as vascular injury, hemodynamic disorders, and inflammation. However, several studies have demonstrated that disturbances in the fibroblast growth factor-23/α-Klotho axis do not lead to cardiac hypertrophy. In this review, we describe the cardiac effects of the fibroblast growth factor-23/α-Klotho axis and summarize recent progress in this field. In addition, we present not only the main controversies in this field but also provide possible directions to resolve these disputes.

Iron replacement ameliorates hypophosphatemia in autosomal dominant hypophosphatemic rickets: A review of the role of iron

Autosomal dominant hypophosphatemic rickets (ADHR) is remarkable among the hypophosphatemic rickets syndromes for its variable age of presentation and periods of quiescence during which serum phosphate and fibroblast growth factor 23 (FGF 23) levels are normal without therapy. In contrast, hypophosphatemia in X-linked hypophosphatemic rickets (XLH) manifests soon after birth and requires lifelong therapy. This suggests that there are environmental factors which can alter FGF 23 activity in ADHR but not in XLH. We present an adult with ADHR in whom resolution of hypophosphatemia was achieved by correcting iron deficiency without the need for phosphate supplementation. Serial iron and FGF 23 levels revealed an inverse relationship (r=-0.79, p<0.04). All patients with ADHR who present with hypophosphatemia and worsening symptoms should be screened for iron deficiency. If iron deficiency is detected, therapy with a combination of calcitriol and iron supplementation should be considered without phosphate supplementation.

Physiology of FGF23 and overview of genetic diseases associated with renal phosphate wasting

Phosphate is a cornerstone of several physiological pathways including skeletal development, bone mineralization, membrane composition, nucleotide structure, maintenance of plasma pH, and cellular signaling. The kidneys have a key role in phosphate homeostasis with three hormones having important functions in renal phosphate handling or intestinal absorption: parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1-25-dihydroxyvitamin D (1,25(OH)2D). FGF23 is mainly synthesized by osteocytes; it is a direct phosphaturic factor that also inhibits 1,25(OH)2D and PTH. In addition to crucial effects on phosphate and calcium metabolism, FGF23 also has 'off-target' effects notably on the cardiovascular, immune and central nervous systems. Genetic diseases may affect the FGF23 pathway, resulting in either increased FGF23 levels leading to hypophosphatemia (such as in X-linked hypophosphatemia) or defective secretion/action of intact FGF23 inducing hyperphosphatemia (such as in familial tumoral calcinosis). The aim of this review is to provide an overview of FGF23 physiology and pathophysiology in X-linked hypophosphatemia, with a focus on FGF23-associated genetic diseases.

Evolving Role of Vitamin D in Immune-Mediated Disease and Its Implications in Autoimmune Hepatitis

Vitamin D has immunomodulatory, anti-inflammatory, antioxidant, and anti-fibrotic actions that may impact on the occurrence and outcome of immune-mediated disease. The goals of this review are to describe the nature of these expanded roles, examine the implications of vitamin D deficiency in autoimmune hepatitis, and identify opportunities for future investigation. Abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed. Vitamin D receptors are expressed on the principal cell populations involved in the innate and adaptive immune responses. Macrophages and dendritic cells can produce 1,25-dihydroxyvitamin D within the microenvironment. This active form of vitamin D can inhibit immune cell proliferation, promote an anti-inflammatory cytokine profile, expand regulatory T cells, enhance glucocorticoid actions, increase glutathione production, and inhibit hepatic stellate cells. Vitamin D deficiency has been commonly present in patients with immune-mediated liver and non-liver diseases, and it has been associated with histological severity, advanced hepatic fibrosis, and non-response to conventional glucocorticoid therapy in autoimmune hepatitis. Vitamin D analogues with high potency, low calcemic effects, and independence from hepatic hydroxylation are possible interventions. In conclusion, vitamin D has properties that could ameliorate immune-mediated disease, and vitamin D deficiency has been a common finding in immune-mediated liver and non-liver diseases, including autoimmune hepatitis. Loss of vitamin D-dependent homeostatic mechanisms may promote disease progression. Vitamin D analogues that are independent of hepatic hydroxylation constitute an investigational opportunity to supplement current management of autoimmune hepatitis.

Technical and diagnostic performance of a new fully automated immunoassay for the determination of intact fibroblast growth factor 23 (FGF23)

There is growing interest in measuring plasma fibroblast growth factor 23 (FGF23) concentrations in a number of clinical settings. However, a reliable assay with acceptable performance is lacking. Plasma samples of healthy adults and patients with different stages of chronic kidney disease (CKD) were used to compare the precision, recovery, linearity and the pre-analytical stability characteristics of a new fully automated FGF23 (intact) assay with a commercially available FGF23 (intact) ELISA. Method agreement was evaluated, reference and stage-specific ranges for kidney disease were established. Other biomarkers relevant for CKD were measured and compared with the FGF23 assays. The fully automated FGF23 (intact) assay demonstrated superior performance compared with the ELISA. A marked positive proportional bias was detected relative to the ELISA assay readout, especially in samples of higher concentration of patients undergoing hemodialysis. Overall, the method comparison revealed a poor degree of correlation. A significant inverse correlation was found between the glomerular filtration rate and both FGF23 assays (both p < .001). Regression analysis revealed that both assays are suitable to predict progression of CKD. A positive correlation was found between FGF23 and phosphate, parathyroid hormone (PTH) and vitamin D, 25(OH)D and 1,25(OH)₂D-total assays, respectively. Cutoff points between different stages of CKD were calculated by receiver operator characteristic analysis. The fully automated assay displayed an improved discrimination compared with the ELISA, especially in mild to moderate kidney disease. The new fully automated FGF23 (intact) assay demonstrates excellent analytical performance data and represents a robust, fast and precise alternative to manual FGF23 testing.

Genetic Diseases of Vitamin D Metabolizing Enzymes

Vitamin D metabolism involves 3 highly specific cytochrome P450 (CYP) enzymes (25-hydroxylase, 1α-hydroxylase, and 24-hydroxylase) involved in the activation of vitamin D₃ to the hormonal form, 1,25-(OH)₂D₃, and the inactivation of 1,25-(OH)₂D₃ to biliary excretory products. Mutations of the activating enzymes CYP2R1 and CYP27B1 cause lack of normal 1,25-(OH)₂D₃ synthesis and result in rickets whereas mutations of the inactivating enzyme CYP24A1 cause build-up of excess 1,25-(OH)₂D₃ and result in hypercalcemia, nephrolithiasis, and nephrocalcinosis. This article reviews the literature for 3 clinical conditions. Symptoms, diagnosis, treatment, and management of vitamin D-dependent rickets and idiopathic infantile hypercalcemia are discussed.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome clinically characterized by bone pain, fractures and muscle weakness. It is caused by tumoral overproduction of fibroblast growth factor 23 (FGF23) that acts primarily at the proximal renal tubule, decreasing phosphate reabsorption and 1α-hydroxylation of 25 hydroxyvitamin D, thus producing hypophosphatemia and osteomalacia. Lesions are typically small, benign mesenchymal tumors that may be found in bone or soft tissue, anywhere in the body. In up to 60% of these tumors, a fibronectin-1(FN1) and fibroblast growth factor receptor-1 (FGFR1) fusion gene has been identified that may serve as a tumoral driver. The diagnosis is established by the finding of acquired chronic hypophosphatemia due to isolated renal phosphate wasting with concomitant elevated or inappropriately normal blood levels of FGF23 and decreased or inappropriately normal 1,25-OH₂-Vitamin D (1,25(OH)₂D). Locating the tumor is critical, as complete removal is curative. For this purpose, a step-wise approach is recommended, starting with a thorough medical history and physical examination, followed by functional imaging. Suspicious lesions should be confirmed by anatomical imaging, and if needed, selective venous sampling with measurement of FGF23. If the tumor is not localized, or surgical resection is not possible, medical therapy with phosphate and active vitamin D is usually successful in healing the osteomalacia and reducing symptoms. However, compliance is often poor due to the frequent dosing regimen and side effects. Furthermore, careful monitoring is needed to avoid complications such us secondary/tertiary hyperparathyroidism, hypercalciuria, and nephrocalcinosis. Novel therapeutical approaches are being developed for TIO patients, such as image-guided tumor ablation and medical treatment with the anti-FGF23 monoclonal antibody KRN23 or anti FGFR medications. The case of a patient with TIO is presented to illustrate the importance of adequate and appropriate evaluation of patients with bone pain and hypophosphatemia, as well as an step-wise localization study of patients with suspected TIO.

Tumor-induced osteomalacia: experience from a South American academic center

The majority of tumor-induced osteomalacia cases have been reported in the Northern Hemisphere and Asia. In this first series of South American patients, we show that the clinical presentation and sensitivity of plasmatic fibroblast growth factor 23 and somatostatin analog-based imaging are similar to those described in other populations.

INTRODUCTION

Describe the experience of clinical presentation, diagnostic study, and treatment of patients with tumor-induced osteomalacia (TIO) in a South American academic center in comparison to literature.

METHODS

Analysis of the records of patients diagnosed with TIO. The clinical presentation, diagnostic studies, and treatment were analyzed. Fibroblast growth factor 23 (FGF23) was measured by ELISA.

RESULTS

Six patients were diagnosed with TIO during the studied period. The patients' median age was 53 years (range 22-64). All patients presented with weakness and pain in the extremities. Four experienced fractures during their evolution. The median time to diagnosis was 4.5 years (1-20). Biochemical studies showed hypophosphatemia, median of 1.4 mg/dL (1.2-1.6), with low maximum rates of tubular reabsorption of phosphate adjusted for glomerular filtration rate. FGF23 was elevated in 4/6 patients and inappropriately normal in the other two. In three patients, the location of the tumor was clinically evident and confirmed with anatomical imaging. In the remaining patients, two tumors were located with ⁶⁸Ga DOTATATE-PET/CT and one with OctreoScan. The causal tumors were located in the lower extremities in five patients and invading the frontal sinus in one patient. In all patients, tumors were successfully removed. Within 14 days, there was normalization of phosphate and FGF23 levels and resolution of clinical symptoms in all patients. In all cases, the histopathology was compatible with a phosphaturic mesenchymal tumor.

CONCLUSIONS

The clinical presentation, delay time to diagnosis, FGF23 diagnostic sensitivity and histopathology in this first series of South American patients is similar to those described in other populations. The success of localization by somatostatin analog-based imaging, suggests this may the optimal imaging modality.

Klotho and fibroblast growth factor 23 in cerebrospinal fluid in children

The fibroblast growth factor (FGF) 23/Klotho axis is a principal regulator of phosphate hemostasis and vitamin D metabolism, but limited data is available on its role in the central nervous system. Here, we investigate soluble α-Klotho (sKlotho) and C-terminal as well as intact FGF23 in cerebrospinal fluid (CSF) and plasma and their relationship to mineral metabolism parameters in humans. In 39 children aged 0.3-16.8 years undergoing lumbar puncture for the exclusion of inflammatory neurological disease, sKlotho and FGF23 were investigated by Western blot analysis, followed by ELISA quantification in CSF and plasma. The percentage of intrathecal synthesis of both proteins was calculated by measuring both the expected and observed CSF/plasma ratios of sKlotho and FGF23. The secreted (KL1) and cleaved (KL1+KL2) isoforms of sKlotho, and FGF23 were clearly detected in CSF in all subjects, although protein levels were lower compared to those of plasma samples (each p < 0.01). The intrathecal percentage of CSF sKlotho and FGF23 synthesis amounted to 98 and 99 %, respectively. CSF sKlotho levels were higher in boys than in girls (p < 0.01), and correlated positively with plasma C-terminal FGF23 concentrations (p < 0.05) and standardized height (p < 0.01). Importantly, there were no significant correlations between plasma and CSF levels of sKlotho or FGF23. Plasma sKlotho as well as C-terminal and intact FGF23, respectively, were associated with parameters of mineral metabolism These results provide evidence that cleaved and secreted sKlotho and FGF23 are present in CSF, mainly derived from brain and affected by sex, height, and mineral metabolism parameters in children. Nevertheless, the absence of significant associations between plasma and CSF levels of Klotho and FGF23, respectively, suggest that the regulation of Klotho and FGF23 may be different between organs secreting these hormones into blood and CSF.

Bone-associated gene evolution and the origin of flight in birds

BACKGROUND

Bones have been subjected to considerable selective pressure throughout vertebrate evolution, such as occurred during the adaptations associated with the development of powered flight. Powered flight evolved independently in two extant clades of vertebrates, birds and bats. While this trait provided advantages such as in aerial foraging habits, escape from predators or long-distance travels, it also imposed great challenges, namely in the bone structure.

RESULTS

We performed comparative genomic analyses of 89 bone-associated genes from 47 avian genomes (including 45 new), 39 mammalian, and 20 reptilian genomes, and demonstrate that birds, after correcting for multiple testing, have an almost two-fold increase in the number of bone-associated genes with evidence of positive selection (~52.8 %) compared with mammals (~30.3 %). Most of the positive-selected genes in birds are linked with bone regulation and remodeling and thirteen have been linked with functional pathways relevant to powered flight, including bone metabolism, bone fusion, muscle development and hyperglycemia levels. Genes encoding proteins involved in bone resorption, such as TPP1, had a high number of sites under Darwinian selection in birds.

CONCLUSIONS

Patterns of positive selection observed in bird ossification genes suggest that there was a period of intense selective pressure to improve flight efficiency that was closely linked with constraints on body size.

Klotho, FGF21 and FGF23: Novel Pathways to Musculoskeletal Health?

Bone mineral density, muscle mass and physical function reach their peak between the second and fourth decade of life and then decline steadily with aging. The crucial question is: what factors contribute to or modulate this decline? The aim of this mini-review is to propose a theoretical framework for the potential role of emerging biomarkers such as klotho, fibroblast growth factors (FGF)21 and FGF23 on musculoskeletal health, with a particular focus on decline in muscle mass and function, and calls for future research to examine this proposed link. The identification of new physiological mechanisms underlying these declines may open a potentially important avenue for the development of novel intervention strategies aimed at preventing or reducing their potentially detrimental consequences.

The effect of GH treatment on serum FGF23 and Klotho in GH-deficient children

BACKGROUND

Normal phosphate homeostasis is essential for normal linear growth. The phosphaturic fibroblast growth factor 23 (FGF23)/Klotho axis is a major regulator of phosphate homeostasis; therefore, an intact FGF23/Klotho axis is important for normal linear growth. On the other hand, GH/IGF1 axis has opposing effects on phosphate homeostasis, but the underline mechanisms remain unclear.

AIM

The main objective of this study was to investigate the possible interactions of FGF23 and its co-receptor Klotho, with growth hormone (GH)/IGF1 axis in the regulation of phosphate metabolism in GH-deficient children under GH treatment.

METHODS

We studied 23 GH-deficient children, before and 3 months after the onset of GH treatment. Anthropometry and assessment of biochemical parameters were performed, as well as measurement of FGF23 (intact FGF23/iFGF23 and C-terminal FGF23/cFGF23) and soluble α-Klotho (sKlotho) levels.

RESULTS

After 3 months on GH treatment, the elevation of serum phosphate and TmPO4/GFR (P<0.0001 and P<0.01 respectively) was accompanied by a significant increase in cFGF23 (P<0.01), iFGF23 (P<0.0001), sKlotho (P<0.0001) and IGF1 (P<0.0001). Serum phosphate and TmPO4/GFR were positively associated with iFGF23 (P<0.01 and P<0.05) and IGF1 (P<0.05 and P<0.05). iFGF23 levels were positively correlated with sKlotho (P<0.001), IGF1 (P<0.0001) and height SDS (P<0.0001), whereas sKlotho was positively associated with IGF1 (P<0.0001) and height SDS (P<0.001).

CONCLUSION

The increase in serum phosphate, which we found in GH-deficient children under GH treatment, is not associated with suppression but rather than with upregulation of the phosphaturic FGF23/Klotho axis.

Breast milk stimulates growth hormone secretion in infant mice, and phosphorus insufficiency disables this ability and causes dwarfism-like symptoms

INTRODUCTION

Breast milk intake facilitates neonatal growth, and its effect is assumed to last long into the adulthood. We recently reported that dietary phosphorus insufficiency reduces the ability of breast milk to promote infant growth in mice. However, how phosphorus confers this ability to milk is still unclear.

METHODS

To address this issue, we performed biochemical and physiological comparisons of milk secreted from C57BL/6J mice fed a low-phosphorus diet (LPD) or a normal-phosphorus control diet.

RESULTS

Although serum phosphorus concentration was decreased, the body weight of mother mice was unaffected. By contrast, infant body weight was significantly reduced, and dwarfism-like symptoms were observed in adulthood. Quantitative analysis revealed that the serum concentration of growth hormone (GH) was substantially reduced, and concomitantly insulin-like growth factor 1 and fibroblast growth factor 23 were decreased. Immunohistochemical analysis revealed ectopic fat accumulation in the livers of infant mice along with increased blood cholesterol level. Moreover, electron microscopy indicated fragility of the outer membrane of milk droplets.

CONCLUSIONS

Our results suggest that phosphorus is essential for the formation of milk droplets, which function as a stimulator of growth factor secretion in infant offspring.

Identification of a Mutation in FGF23 Involved in Mandibular Prognathism

Mandibular prognathism (MP) is a severe maxillofacial disorder with undetermined genetic background. We collected a Chinese pedigree with MP which involved in 23 living members of 4 generations. Genome-wide linkage analysis were carried out to obtain the information in this family and a new MP-susceptibility locus, 12pter-p12.3 was identified. Whole-exome sequencing identified a novel heterozygous mutation in fibroblast growth factor (FGF) 23 (; p.A12D) which well segregated with MP in this pedigree within the locus. The mutation was also detected in 3 cases out of 65 sporadic MP patients, but not in any of the 342 control subjects. The p.A12D mutation may disrupt signal peptide function and inhibit secretory in FGF23. Furthermore, mutant FGF23 was overexpressed in 293T cells, increased cytoplasmic accumulation was observed compared with the wild type. We have discovered that c.35C>A mutation in FGF23 strongly associated with MP, which expand our understanding of the genetic contribution to MP pathogenesis.

Hyperphosphatemic familial tumoral calcinosis: genetic models of deficient FGF23 action

Hyperphosphatemic familial tumoral calcinosis (hFTC) is a rare disorder of phosphate metabolism defined by hyperphosphatemia and ectopic calcifications in various locations. To date, recessive mutations have been described in three genes involving phosphate metabolism: FGF23, GALNT3, and α-Klotho, all of which result in the phenotypic presentation of hFTC. These mutations result in either inadequate intact fibroblast growth factor-23 (FGF23) secretion (FGF23 or GALNT3) or resistance to FGF23 activity at the fibroblast growth factor receptor/α-Klotho complex (α-Klotho). The biochemical consequence of limitations in FGF23 activity includes increased renal tubular reabsorption of phosphate, hyperphosphatemia, and increased production of 1,25-dihydroxyvitamin D. The resultant ectopic calcifications can be painful and debilitating. Medical treatments are targeted toward decreasing intestinal phosphate absorption or increasing phosphate excretion; however, results have been variable and generally limited. Treatments that would increase FGF23 levels or signaling would more appropriately target the genetic etiologies of this disease and perhaps be more effective.

Fibroblast growth factor 23 and Klotho serum levels in healthy children

Data for fibroblast growth factor 23 (FGF23) and particularly for Klotho in healthy children are limited. We aimed to investigate the relationship between FGF23 and Klotho with age and TmP/GFR and to evaluate parameters that might affect FGF23 and Klotho. In 159 healthy children (82 boys) with a mean±SD age of 8.78±3.47years we measured FGF23 (intact FGF23/iFGF23 and C-terminal FGF23/cFGF23) and soluble aKlotho serum levels by ELISA. Mean±SD value for cFGF23, was 51.14±12.79 RU/ml whereas median (range) values for iFGF23 and Klotho were 35 (8.8, 120) pg/ml and 1945 (372, 5866) pg/ml respectively. Neither FGF23 nor Klotho were significantly associated with age. Pubertal children had higher Klotho than prepubertal (p<0.05), and girls had higher levels of cFGF23 (p<0.05) and Klotho (p<0.001) than boys. Serum phosphate and TmP/GFR were positively associated with cFGF23 (p<0.01 and p<0.001), iFGF23 (p<0.05 and p<0.001) and Klotho (p<0.05 and p<0.01). Klotho was positively correlated with IGF-I (p<0.0001) and 1,25 (OH)2 vitamin D (p<0.05). In this study we provide data on cFGF23, iFGF23, and Klotho measured simultaneously in healthy children. The positive association of serum phosphate and TmP/GFR with FGF23 and Klotho suggests that they have a counterregulatory effect on phosphate homeostasis. The strong association of Klotho with IGF-I could indicate a role of Klotho in linear growth through phosphate regulation, but further studies are required.

Differential response of idiopathic sporadic tumoral calcinosis to bisphosphonates

CONTEXT

Tumoral calcinosis is a disorder of phosphate metabolism characterized by ectopic calcification around major joints. Surgery is the current treatment of choice, but a suboptimal choice in recurrent and multicentric lesions.

AIMS

To evaluate the efficacy of bisphosphonates for the management of tumoral calcinosis on optimized medical treatment.

SETTINGS AND DESIGN

The study was done in the endocrine department of a tertiary care hospital in South India. We prospectively studied two patients with recurrent tumoral calcinosis who had failed therapy with phosphate lowering measures.

MATERIALS AND METHODS

After informed consent, we treated both patients with standard age adjusted doses of bisphosphonates for 18 months. The response was assessed by X ray and whole body 99mTc-methylene diphosphonate bone scan at the beginning of therapy and at the end of 1 year. We also estimated serum phosphate levels and urinary phosphate to document serial changes.

RESULTS

Two patients (aged 19 and 5 years) with recurrent idiopathic hyperphosphatemic tumoral calcinosis, following surgery were studied. Both patients had failed therapy with conventional medical management - low phosphate diet and phosphate binders. They had restriction of joint mobility. Both were given standard doses of oral alendronate and parenteral pamidronate respectively for more than a year, along with phosphate lowering measures. At the end of 1 year, one of the patients had more than 95% and 90% reduction in the size of the lesions in right and left shoulder joints respectively with total improvement in range of motion. In contrast, the other patient (5-year-old) had shown no improvement, despite continuing to maintain normophosphatemia following treatment.

CONCLUSIONS

Bisphosphonate therapy in tumoral calcinosis is associated with lesion resolution and may be used as a viable alternative to surgery, especially in cases with multicentric recurrence or treatment failure to other drugs.

Cytochrome P450-mediated metabolism of vitamin D

The vitamin D signal transduction system involves a series of cytochrome P450-containing sterol hydroxylases to generate and degrade the active hormone, 1α,25-dihydroxyvitamin D3, which serves as a ligand for the vitamin D receptor-mediated transcriptional gene expression described in companion articles in this review series. This review updates our current knowledge of the specific anabolic cytochrome P450s involved in 25- and 1α-hydroxylation, as well as the catabolic cytochrome P450 involved in 24- and 23-hydroxylation steps, which are believed to initiate inactivation of the vitamin D molecule. We focus on the biochemical properties of these enzymes; key residues in their active sites derived from crystal structures and mutagenesis studies; the physiological roles of these enzymes as determined by animal knockout studies and human genetic diseases; and the regulation of these different cytochrome P450s by extracellular ions and peptide modulators. We highlight the importance of these cytochrome P450s in the pathogenesis of kidney disease, metabolic bone disease, and hyperproliferative diseases, such as psoriasis and cancer; as well as explore potential future developments in the field.

FGF-23: the rise of a novel cardiovascular risk marker in CKD

Elevated plasma levels of the phosphaturic hormone fibroblast growth factor 23 (FGF-23) are a hallmark of chronic kidney disease (CKD)-mineral and bone disorder. FGF-23 allows serum phosphate levels within physiological limits to be maintained in progressive CKD until end-stage renal disease is reached. Despite its seemingly beneficial role in phosphate homeostasis, several prospective studies in dialysis patients and in patients with less advanced CKD associated elevated FGF-23 with poor cardiovascular and renal outcome. Moreover, very recent evidence suggests an adverse prognostic impact of elevated FGF-23 even in subjects without manifest CKD. These epidemiological data are supplemented by laboratory findings that reveal a pathophysiological role of FGF-23 in the pathogenesis of myocardial injury. In aggregate, these clinical and experimental data identify FGF-23 as a promising target of novel therapeutic interventions in CKD and beyond, which should be tested in future clinical trials.

Anesthesia for oncogenic osteomalacia--a rare paraneoplastic syndrome

Two patients with a diagnosis of oncogenic osteomalacia are described. This rare disease, characterized by secretion of fibroblast growth factor-23 by the tumor cells, causes myopathy, extreme debilitation and severe osteopathy because of severe hypophosphatemia. Both patients presented with severe bone pain, pathological fractures and proximal muscle weakness. Multiple diagnostic tools had to be utilized to settle the diagnosis of this rare disease. Although supplemental therapy for hypophosphatemia is usually started preoperatively, surgical excision of the causative tumor is the only definite treatment. Surgery is almost always curative; however, there is a lack of discourse in the literature regarding the anesthetic implications for the disease. The complete pathophysiology of the disease, clinical picture, its diagnostic intricacies as well as the salient points in its anesthetic management are discussed in this report.

A mouse with an N-Ethyl-N-nitrosourea (ENU) Induced Trp589Arg Galnt3 mutation represents a model for hyperphosphataemic familial tumoural calcinosis

Mutations of UDP-N-acetyl-alpha-D-galactosamine polypeptide N-acetyl galactosaminyl transferase 3 (GALNT3) result in familial tumoural calcinosis (FTC) and the hyperostosis-hyperphosphataemia syndrome (HHS), which are autosomal recessive disorders characterised by soft-tissue calcification and hyperphosphataemia. To facilitate in vivo studies of these heritable disorders of phosphate homeostasis, we embarked on establishing a mouse model by assessing progeny of mice treated with the chemical mutagen N-ethyl-N-nitrosourea (ENU), and identified a mutant mouse, TCAL, with autosomal recessive inheritance of ectopic calcification, which involved multiple tissues, and hyperphosphataemia; the phenotype was designated TCAL and the locus, Tcal. TCAL males were infertile with loss of Sertoli cells and spermatozoa, and increased testicular apoptosis. Genetic mapping localized Tcal to chromosome 2 (62.64-71.11 Mb) which contained the Galnt3. DNA sequence analysis identified a Galnt3 missense mutation (Trp589Arg) in TCAL mice. Transient transfection of wild-type and mutant Galnt3-enhanced green fluorescent protein (EGFP) constructs in COS-7 cells revealed endoplasmic reticulum retention of the Trp589Arg mutant and Western blot analysis of kidney homogenates demonstrated defective glycosylation of Galnt3 in Tcal/Tcal mice. Tcal/Tcal mice had normal plasma calcium and parathyroid hormone concentrations; decreased alkaline phosphatase activity and intact Fgf23 concentrations; and elevation of circulating 1,25-dihydroxyvitamin D. Quantitative reverse transcriptase-PCR (qRT-PCR) revealed that Tcal/Tcal mice had increased expression of Galnt3 and Fgf23 in bone, but that renal expression of Klotho, 25-hydroxyvitamin D-1α-hydroxylase (Cyp27b1), and the sodium-phosphate co-transporters type-IIa and -IIc was similar to that in wild-type mice. Thus, TCAL mice have the phenotypic features of FTC and HHS, and provide a model for these disorders of phosphate metabolism.

Review article: vitamin D and inflammatory bowel disease--established concepts and future directions

BACKGROUND

Understanding of the role of vitamin D in health and disease has increased markedly in the past decade, with its involvement extending well beyond traditional roles in calcium and phosphate homeostasis and musculoskeletal health. This conceptual expansion has been underpinned by identification and exploration of components of this axis including vitamin D-binding protein, key enzymes and receptors in multiple cell types, and a greater recognition of nonclassical autocrine and paracrine effects. Its influence in IBD remains uncertain.

AIM

To review the role of vitamin D in bone health, immune regulation and cancer prevention in IBD, and to outline practical issues and limitations of its use.

METHODS

An extensive online literature review including PubMed and Medline.

RESULTS

In patients with IBD, the vitamin D axis provides an important and often underutilised pathway to preserving bone health. Furthermore, an exciting body of clinical and basic science research demonstrates that these pathways may have an integral part to play in regulation of the immune response in IBD, through effects on the intestinal barrier, antigen presenting cells and adaptive T cells. The possibility of chemoprevention requires further study. The optimal target level of 25-hydroxy vitamin D in patients with IBD is currently uncertain, as is the best therapeutic modality.

CONCLUSIONS

Study of vitamin D pathways may result in the development of relatively inexpensive therapeutic options to optimise patient outcomes. Further prospective clinical research is required to address efficacy and long-term safety.

Uric acid and IGF1 as possible determinants of FGF23 metabolism in children with normal renal function

BACKGROUND

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone and a suppressor of renal 1α hydroxylase. Although circulating values of FGF23 are increased in early chronic kidney disease (CKD), the interplay between FGF23 levels, growth and nutritional biomarkers has not been evaluated in children with normal renal function.

METHODS

We performed a secondary analysis of the cross-sectional observational INU23 study in 98 children (51 boys, mean age 10.5 ± 3.9 years) with preserved renal function (glomerular filtration rate (GFR) 114 ± 14 ml/min/1.73 m(2)).

RESULTS

In bivariate analyses, C-terminal FGF23 levels were positively related to phosphorus and uric acid levels. Intact FGF23 levels were positively associated with uric acid and insulin growth factor 1 (IGF1) levels, with similar results for age, body mass index (BMI), and 25OH vitamin D (25(OH)D). By multivariable analyses, 25(OH)D, uric acid, and phosphorus were independent predictors of C-terminal FGF23, while 25(OH)D, uric acid, and IGF1 were independent predictors of intact FGF23.

CONCLUSIONS

In children with preserved kidney function, the association between FGF23, uric acid, and IGF1 suggests that FGF23 could be an early nutritional indicator of high protein and phosphate intake. The association between FGF23 and IGF1 also suggests a relationship between FGF23 and growth, and warrants further investigation.

Fibroblast growth factor 23 is associated with proteinuria and smoking in chronic kidney disease: an analysis of the MASTERPLAN cohort

BACKGROUND

Fibroblast growth factor 23 (FGF23) has emerged as a risk factor for cardiovascular disease and mortality throughout all stages of chronic kidney disease (CKD), independent from established risk factors and markers of mineral homeostasis. The relation of FGF23 with other renal and non-renal cardiovascular risk factors is not well established.

METHODS

Using stored samples, plasma FGF23 was determined in 604 patients with moderate to severe kidney disease that participated in the MASTERPLAN study (ISRCTN73187232). The association of FGF23 with demographic and clinical parameters was evaluated using multivariable regression models.

RESULTS

Mean age in the study population was 60 years and eGFR was 37 (± 14) ml/min/1.73 m(2). Median proteinuria was 0.3 g/24 hours [IQR 0.1-0.9]. FGF23 level was 116 RU/ml [67-203] median and IQR. Using multivariable analysis the natural logarithm of FGF23 was positively associated with history of cardiovascular disease (B = 0.224 RU/ml; p = 0.002), presence of diabetes (B = 0.159 RU/ml; p = 0.035), smoking (B = 0.313 RU/ml; p < 0.001), phosphate level (B = 0.297 per mmol/l; p = 0.0024), lnPTH (B = 0.244 per pmol/l; p < 0.001) and proteinuria (B = 0.064 per gram/24 hrs; p = 0.002) and negatively associated with eGFR (B = -0.022 per ml/min/1.73 m(2); p < 0.001).

CONCLUSIONS

Our study demonstrates that in patients with CKD, FGF23 is related to proteinuria and smoking. We confirm the relation between FGF23 and other cardiovascular risk factors.

1,25-Dihydroxyvitamin D(3) and extracellular inorganic phosphate activate mitogen-activated protein kinase pathway through fibroblast growth factor 23 contributing to hypertrophy and mineralization in osteoarthritic chondrocytes

Hypertrophy and impaired mineralization are two processes closely associated with osteoarthritis (OA). 1,25-dihydroxyvitamin D(3) (1a,25(OH)(2)D(3)) and inorganic phosphate (Pi) are two important factors that are implicated in calcium and phosphate homeostasis of bone metabolism and both can be regulated by the circulating phosphaturic factor fibroblast growth factor 23 (FGF23). The objective of this study was to investigate the role of 1a,25(OH)(2)D(3) and Pi and the molecular mechanism through which they contribute to hypertrophy and mineralization in human osteoarthritic chondrocytes. For this purpose, primary human chondrocytes were obtained from articular cartilage which was collected after total knee replacement surgery in OA patients. FGF23, fibroblast growth factor receptor 1c (FGFR1c), vitamin D(3) receptor (VDR), and phosphate inorganic transporter-1 and -2 (PiT-1 and PiT-2) expression levels were evaluated and found to be significantly higher in OA chondrocytes compared with normal. In addition, we observed that the binding of FGF23 to FGFR1c was stronger in OA chondrocytes compared  with normal. Chromatin immunoprecipitation (ChIP) assay revealed, for the first time, the presence of two vitamin D response elements (VDREs) in the FGF23 promoter. Treatment of normal chondrocytes with 1a,25(OH)(2)D(3) or Pi resulted in significant up-regulation of VDR, FGF23, PiT-1, PiT-2 mRNA and protein levels, extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation and induction of hypertrophy markers collagen type X (COL10A1), osteopontin (OPN), osteocalcin (OC), catabolic markers metalloproteinase-13 (MMP-13) and the apoptotic marker caspase-9. Furthermore, VDR silencing in OA chondrocytes negatively regulated FGF23, COL10A1, OPN, OC, MMP-13 and caspase-9 expressions and ERK1/2 phosphorylation. Finally, combined VDR silencing and PiT-1, PiT-2 inhibition in OA chondrocytes resulted in additive down-regulation of FGF23 expression, ERK1/2 activation and COL10A1, OPN, OC, MMP-13 and caspase-9 expression levels. We propose that 1a,25(OH)(2)D(3) and Pi act synergistically through FGF23 signaling and ERK1/2 phosphorylation contributing to late hypertrophic events and impaired mineralization in osteoarthritic chondrocytes.

The structural biology of the FGF19 subfamily

The ability of the Fibroblast Growth Factor (FGF) 19 subfamily to signal in an endocrine fashion sets this subfamily apart from the remaining five FGF subfamilies known for their paracrine functions during embryonic development. Compared to the members of paracrine FGF subfamiles, the three members of the FGF19 subfamily, namely FGF19, FGF21 and FGF23, have poor affinity for heparan sulfate (HS) and therefore can diffuse freely in the HS-rich extracellular matrix to enter into the bloodstream. In further contrast to paracrine FGFs, FGF19 subfamily members have unusually poor affinity for their cognate FGF receptors (FGFRs) and therefore cannot bind and activate them in a solely HS-dependent fashion. As a result, the FGF19 subfamily requires α/βklotho coreceptor proteins in order to bind, dimerize and activate their cognate FGFRs. This klotho-dependency also determines the tissue specificity of endocrine FGFs. Recent structural and biochemical studies have begun to shed light onto the molecular basis for the klotho-dependent endocrine mode of action of the FGF19 subfamily. Crystal structures of FGF19 and FGF23 show that the topology of the HS binding site (HBS) of FGF19 subfamily members deviates drastically from the common topology adopted by the paracrine FGFs. The distinct topologies of the HBS of FGF19 and FGF23 prevent HS from direct hydrogen bonding with the backbone atoms of the HBS of these ligands and accordingly decrease the HS binding affinity of this subfamily. Recent biochemical data reveal that the ?klotho ectodomain binds avidly to the ectodomain of FGFR1c, the main cognate FGFR of FGF23, creating a de novo high affinity binding site for the C-terminal tail of FGF23. The isolated FGF23 C-terminus can be used to effectively inhibit the formation of the FGF23-FGFR1c-αklotho complex and alleviate hypophosphatemia in renal phosphate disorders due to elevated levels of FGF23.

25-Hydroxyvitamin D-24-hydroxylase (CYP24A1): its important role in the degradation of vitamin D

CYP24A1 is the cytochrome P450 component of the 25-hydroxyvitamin D(3)-24-hydroxylase enzyme that catalyzes the conversion of 25-hydroxyvitamin D(3) (25-OH-D(3)) and 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) into 24-hydroxylated products, which constitute the degradation of the vitamin D molecule. This review focuses on recent data in the CYP24A1 field, including biochemical, physiological and clinical developments. Notable among these are: the first crystal structure for rat CYP24A1; mutagenesis studies which change the regioselectivity of the enzyme; and the finding that natural inactivating mutations of CYP24A1 cause the genetic disease idiopathic infantile hypercalcemia (IIH). The review also discusses the emerging correlation between rising serum phosphate/FGF-23 levels and increased CYP24A1 expression in chronic kidney disease, which in turn underlies accelerated degradation of both serum 25-OH-D(3) and 1,25-(OH)(2)D(3) in this condition. This review concludes by evaluating the potential clinical utility of blocking this enzyme with CYP24A1 inhibitors in various disease states.

Resolution of severe oncogenic hypophosphatemic osteomalacia after resection of a deeply located soft-tissue tumour

Oncogenic osteomalacia is a rare metabolic bone disease characterized by phosphate leakage from the kidney and subsequent hypophosphatemia. It is caused by a phosphaturic factor produced by certain tumours. Removal of such tumours can completely cure the condition. Here, we report the case of a patient who was crippled with oncogenic osteomalacia. Extensive study revealed a tumour deeply located in the pelvis; removal of the tumour resulted in complete recovery. The tumour was identified as a mesenchymal tumour (mixed connective-tissue variant). The diagnostic evaluation, differential diagnosis, and treatment are discussed.

Fibroblast growth factor 23 and the bone-vascular axis: lessons learned from animal studies

Calcification of arteries and cardiac valves is observed commonly in dialysis patients and represents a major determinant of the heightened cardiovascular risk observed during chronic kidney disease (CKD) progression. Recent advances from clinical and basic science studies suggest that vascular calcification should be considered a systemic disease in which pathologic processes occurring in the bone and kidney contribute to calcium deposition in the vasculature. Among the factors potentially involved in the vascular-bone axis dysregulation associated with CKD, there now is increasing interest in the role of the phosphaturic hormone fibroblast growth factor 23 (FGF-23). Increased FGF-23 plasma levels are observed with a decrease in kidney function and predict the risk of future cardiovascular mortality. However, clinical data are still unclear about whether a direct pathogenetic effect of FGF-23 on vascular/kidney/bone health exists. In the last few years, a series of basic science studies, performed using engineered mice, have contributed important pathophysiologic information about FGF-23 activities. This review summarizes findings from these studies and discusses the potential role of FGF-23 during the pathologic interplay between kidney, vessels, and bone in CKD.

CYP24A1 and kidney disease

PURPOSE OF REVIEW

Patients with chronic renal disease have elevated serum phosphate levels, elevated fibroblast-like growth factor 23 (FGF-23), and declining vitamin D status. These changes are related and may be responsible for elevated 25-hydroxyvitamin D-24-hydroxylase (CYP24A1) and dysfunctional vitamin D metabolism. This review focuses on the biochemistry and pathophysiology of CYP24A1 and the utility of blocking this enzyme with CYP24A1 inhibitors in chronic kidney disease (CKD) patients.

RECENT FINDINGS

CYP24A1 is the cytochrome P450 enzyme that catalyzes the conversion of 25-hydroxyvitamin D3 (25-OHD3) and its hormonal form, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], into 24-hydroxylated products targeted for excretion. The CYP24A1-null phenotype is consistent with the catabolic role of CYP24A1. A number of polymorphisms of CYP24A1 have recently been identified. New data from the uremic rat and humans suggest that dysfunctional vitamin D metabolism is due to changes in CYP24A1 expression caused by phosphate and FGF-23 elevations.

SUMMARY

Changes in serum phosphate and FGF-23 levels in the CKD patient increase CYP24A1 expression resulting in decreased vitamin D status. Vitamin D deficiency may exacerbate defective calcium and phosphate homeostasis causing renal osteodystrophy and contribute to the other complications of renal disease. These findings argue for increased focus on correcting vitamin D deficiency in CKD patients by blocking CYP24A1 activity.

Novel mutations in GALNT3 causing hyperphosphatemic familial tumoral calcinosis

Hyperphosphatemic familial tumoral calcinosis (HFTC) is known to be caused by mutations in at least three genes: FGF23, GALNT3 and KL. Two families with two affected members suffering from HFTC were scrutinized for mutations in these candidate genes. We identified in both families homozygous missense mutations affecting highly conserved amino acids in GALNT3. One of the mutations is a novel mutation, whereas the second mutation was reported before in a compound heterozygous state. Our data expand the spectrum of known mutations in GALNT3 and contribute to a better understanding of the phenotypic manifestations of mutations in this gene.

Vitamin D and immune function: an overview

Immunomodulatory actions of vitamin D have been recognised for over a quarter of a century, but it is only in the last few years that the significance of this to normal human physiology has become apparent. Two key factors have underpinned this revised perspective. Firstly, there are increasing data linking vitamin insufficiency with prevalent immune disorders. Improved awareness of low circulating levels of precursor 25-hydroxyvitamin D in populations across the globe has prompted epidemiological investigations of health problems associated with vitamin D insufficiency. Prominent among these are autoimmune diseases such as multiple sclerosis, type 1 diabetes and Crohn's disease, but more recent studies indicate that infections such as tuberculosis may also be linked to low 25-hydroxyvitamin D levels. The second factor expanding the link between vitamin D and the immune system is our improved knowledge of the mechanisms that facilitate this association. It is now clear that cells from the immune system contain all the machinery needed to convert 25-hydroxyvitamin D to active 1,25-dihydroxyvitamin D, and for subsequent responses to 1,25-dihydroxyvitamin D. Mechanisms such as this are important for promoting antimicrobial responses to pathogens in macrophages, and for regulating the maturation of antigen-presenting dendritic cells. The latter may be a key pathway by which vitamin D controls T-lymphocyte (T-cell) function. However, T-cells also exhibit direct responses to 1,25-dihydroxyvitamin D, notably the development of suppressor regulatory T-cells. Collectively these observations suggest that vitamin D is a key factor linking innate and adaptive immunity, and both of these functions may be compromised under conditions of vitamin D insufficiency.

A better anti-diabetic recombinant human fibroblast growth factor 21 (rhFGF21) modified with polyethylene glycol

As one of fibroblast growth factor (FGF) family members, FGF21 has been extensively investigated for its potential as a drug candidate to combat metabolic diseases. In the present study, recombinant human FGF21 (rhFGF21) was modified with polyethylene glycol (PEGylation) in order to increase its in vivo biostabilities and therapeutic potency. At N-terminal residue rhFGF21 was site-selectively PEGylated with mPEG20 kDa-butyraldehyde. The PEGylated rhFGF21 was purified to near homogeneity by Q Sepharose anion-exchange chromatography. The general structural and biochemical features as well as anti-diabetic effects of PEGylated rhFGF21 in a type 2 diabetic rat model were evaluated. By N-terminal sequencing and MALDI-TOF mass spectrometry, we confirmed that PEG molecule was conjugated only to the N-terminus of rhFGF21. The mono-PEGylated rhFGF21 retained the secondary structure, consistent with the native rhFGF21, but its biostabilities, including the resistance to physiological temperature and trypsinization, were significantly enhanced. The in vivo immunogenicity of PEGylated rhFGF21 was significantly decreased, and in vivo half-life time was significantly elongated. Compared to the native form, the PEGylated rhFGF21 had a similar capacity of stimulating glucose uptake in 3T3-L1 cells in vitro, but afforded a significantly long effect on reducing blood glucose and triglyceride levels in the type 2 diabetic animals. These results suggest that the PEGylated rhFGF21 is a better and more effective anti-diabetic drug candidate than the native rhFGF21 currently available. Therefore, the PEGylated rhFGF21 may be potentially applied in clinics to improve the metabolic syndrome for type 2 diabetic patients.